Metazoan Complexity and Evolution: Is There a Trend?

The notion that complexity increases in evolution is widely accepted, but the best-known evidence is highly impressionistic. In this paper, I propose a scheme for understanding complexity which provides a conceptual basis for objective measurement. The scheme also shows complexity to be a composite term covering four independent types. For each type, I describe some of the measures that have been devised and review the evidence for trends in the maximum and mean. In metazoans as a whole, there is good evidence for an early-Phanerozoic trend, and only in one type of complexity. For each of the other types, some trends have been documented but only in a small number of metazoan subgroups.